1. Field of the Invention
The present invention relates to a split type optical pickup device comprising a fixed optical system and a movable optical system.
The present invention also relates to a split type optical pickup device which writes information on an optical information recording medium and reads information therefrom and which device has a focus servo system and a track servo system which are operated by using reflection rays reflected from the optical information recording medium.
2. Description of the Related Art
An optical pickup device is used for writing information on an optical disk and reading information from the disk. Such an optical pickup device comprises tracking means for conducting a tracking function, that is, for positioning an optical spot on a desired track on the disk. The tracking means comprises a semiconductor laser source, a tracking mirror and an optical focussing head. The semiconductor laser beam emitted from the laser source is deflected by the tracking mirror so that the optical path thereof is adjusted. The deflected beam passes through the focussing head which focuses the beam on the disk at a desired position.
Japanese Patent Application Laying Open (KOKAI) No. 56-187869 discloses an optical pickup device which comprises a two-step positioning system composed of a first rough positioning system and a second accurate positioning system which utilizes the above-mentioned tracking means. The two-step positioning system aims to achieve a high speed access motion.
However, the positioning function is often disturbed when the two positioning systems mechanically interfere with each other. Also, there is another problem that due to weight imbalance about the rotary tracking mirror axis, a rotary moment about the axis is generated in response to the motion of the rough positioning system, which causes an error of the positioning operation of the tracking mirror.
In order to obviate the above-mentioned problems, Japanese Patent Application Laying Open (KOKAI) No. 60-163235 discloses an optical structure wherein the rotary axis of the tracking mirror is disposed in parallel to the direction of the movement of the rough positioning system and wherein an optical system is arranged between the tracking mirror and the focussing system so that the deflection direction of the tracking mirror is turned by 90 degrees.
In accordance with the structure mentioned above, it becomes possible to remove the mechanical interference between the moving pickup and the rotating tracking mirror and avoid the influence due to the weight imbalance of the tracking mirror, which makes it possible to realize a stable control of the spot position as well as shorten the access time by minimizing the vibration of the tracking mirror due to the movement of the pickup at the time of track access operation.
However, the device disclosed in the publication is arranged so that the entire pickup moves in the radial direction of the optical disk and the weight of the movable portion is heavy, which impedes high speed access motion of the pickup. Also, the device requires that the optical system rotates the deflection direction by 90 degrees, which further increases the weight of the pickup and makes it difficult to fully delete the mechanical interference mentioned above and sufficiently shorten the access time of the pickup.
A pickup device is proposed to obviate the drawbacks of the above-mentioned device by adopting an arrangement wherein the tracking mirror is fixed to avoid mechanical interference due to the motion thereof as well as to reduce the weight of the movable portion to achieve a pick up with high speed access motion.
The proposed device comprises a movable optical system and a fixed optical system. The fixed optical system comprises a semiconductor laser source which emits a laser beam and a beam output system comprising a coupling lens, a polarization beam splitter, a quarter-wave plate and a defection prism. The laser beam emitted from the source propagates through the coupling lens, the polarization beam splitter and the quarter-wave plate. The beam is then deflected by the prism toward the tracking mirror which further deflects the beam toward the movable optical system.
The movable optical system reciprocatedly moves in the radial direction of the optical disk. The disk is mounted on a turn table which is fixed on a rotary shaft. The movable optical system comprises a carriage base which is mounted on a guide rail and reciprocatedly movable along the guide rail in the radial direction of the disk. Also, a deflection prism and an objective lens are mounted on the carriage base. The laser beam output from the fixed optical system enters into the movable optical system wherein the beam is deflected by the deflection prism and after that converged by the objective lens so that the beam forms a minute spot on the optical information recording midium of the disk. The focal point of the objective lens is controlled by a focus actuator. The laser beam is reflected by the disk and propagates back to the fixed optical system through the objective lens and the deflection prism. The reflection beam passes through the tracking mirror, the deflection prism, and the quarter-wave plate in the fixed optical system. After that, the reflection beam is deflected by the polarization beam splitter and propagates through a condenser lens and a cylindrical lens to a four-division element which detects a predetermined information signal from the reflection beam as well as outputs a focus error signal and a track error signal. The focus actuator is driven in response to the focus error signal so that the objective lens is shifted along the optical path of the beam. Also, the tracking mirror is rotated in response to the track error signal to adjust the position of the pickup with respect to the track.
However, in accordance with the proposed structure mentioned above, the deflection prism has to be disposed above or below the tracking mirror so that the optical path is deflected and perpendicularly folded, which makes it difficult to realize a compact and thin optical pickup device.
Another optical pickup device relating to the present invention is described hereinafter.
The device comprises an optical pickup body in which semiconductor laser source, a plurality of mirrors, a galvano mirror, and an objective lens are arranged. A laser beam emitted from the source is converged by the objective lens on the optical disk to form a spot thereon.
A tracking servo method for adjusting the position of the optical spot on the disk is such that, first, the pickup body is roughly positioned at a position corresponding to a desired track of the disk by moving the pickup body in the seeking direction with the use of rollers which are attached to the pickup body, and after that, the galvano mirror which has a rotary axis arranged in the seeking direction is rotated to accurately adjust the position of the track of the disk.
By the arrangement in which the rotary axis of the galvano mirror is disposed in parallel with the seeking direction, it becomes possible to delete mechanical interference between the rough positioning system and the accurate positioning system so that a stable optical spot is irradiated on the disk.
However, the pickup body of the above-mentioned device is heavy so that the body cannot move at a high speed to seek the track, which shortens the access time of the pickup.
To cope with the above-mentioned drawbacks, an optical disk drive is proposed by the applicant of this application in the prior Japanese Patent Application No. 63-227842 which is not published yet.
The proposed disk drive comprises a fixed optical system in which a laser beam emitted from a semiconductor laser source is collimated by a coupling lens and passes through a polarization beam splitter and a quarter-wave plate. After that, a deflection prism deflects the laser beam toward a tracking mirror which reflects the laser beam out of the fixed optical system.
The laser beam output from the fixed optical system is introduced into a movable optical system. The movable optical system comprises a carriage base which is mounted on a rail through rollers which rail is disposed along the seeking direction. A deflection prism is mounted on the carriage base. The beam reflected by the prism is converged by an objective lens on a surface of an optical disk to write information on the disk or read information from the disk.
On the other hand, rays reflected from the disk propagate back through the movable system to the fixed system. In the fixed system, the reflection beam is reflected by the polarization beam splitter toward a signal detection portion which includes a condenser lens for converging the beam, a cylindrical lens for generating astigmatism in the beam and a four-division element which detects a focus error signal and a track error signal as well as reads information from the disk.
When the focal point on the disk is to be adjusted, a focus actuator arranged in the movable system is driven in response to the focus error signal. Whereas when the tracking position is to be adjusted, the tracking mirror arranged in the fixed system is driven in response to the track error signal. Since the tracking servo system including the tracking mirror for adjusting the tracking position is disposed within the fixed system, the weight of the movable system is reduced, which makes it possible to shorten the access time of pickup and delete mechanical interference which is generated due to the movement of the movable system and impairs the function and effect of the fixed system.
However, in accordance with the above-mentioned separation type pickup device wherein the movable system is separated from the fixed system, the optical path between the deflection prism and the tracking mirror and to the disk surface, is, perpendicular to the optical path between the fixed system and the movable system. Therefore, a relatively large space is required in the direction perpendicular to the disk surface, which means the device becomes tall and large.